The whole numbers from 10 to 99 are positive and have two digits, we can find the amount of them like this:
from 10 to 19->10 numbers
from 20 to 29->10 numbers
from 30 to 39->10 numbers
from 40 to 49->10 numbers
from 50 to 59->10 numbers
from 60 to 69->10 numbers
from 70 to 79->10 numbers
from 80 to 89->10 numbers
from 90 to 99->10 numbers
Since we have 9 groups of 10 numbers, if we add them we get 90.
Then, the number of different positive two-digit whole number is 90